I would use these rules:
(1) if the defender sees a key battle coming and can predict that they’ll want an unusual order of loss, they can leave a note, send an e-mail, etc., informing the attacker of their preferred order of loss, and the attacker will then follow that order.
(2) if there is no note, the attacker will provisionally use a standard order of loss.
(3) after seeing the results of the battle, the defender can pay 2 IPCs to the bank per casualty that they want to switch. You didn’t want to lose your bomber after all? Fine, pay 2 IPCs, and you can have your bomber back in exchange for one of your infantry. This should be expensive enough to deter casual abuse but still cheap enough that if you would really be upset about the order of loss, you can fix it without too much drama. Don’t think it’s fair that you should have to pay to set up your order of loss exactly the way you want it? Well, then, you should have foreseen the battle and given orders in advance. Don’t feel like thinking that hard every turn? Well, then, you can always just cough up 2 IPCs. It’s a way of speeding up the game while making sure that nothing unacceptably bad will happen because of order of loss issues.